Onsen egg
An egg cooked slowly at just under 70 °C, leaving the yolk creamy and the white only half set.
The onsen egg takes its name from Japan's hot springs, where eggs were traditionally cooked. At around 63 to 65 °C the yolk sets before the white, so the result is the reverse of a boiled egg: the yolk turns dense and custard-like while the white stays tender, milky and almost liquid. It is usually cracked over rice, noodles or into broth, where it melts into the dish as a mild, silky sauce. It is made at home with a thermometer and a pot of water held at that temperature for 30 to 45 minutes, or with a sous-vide device. Ready-cooked ones occasionally turn up chilled in Japanese grocery stores or Asian supermarkets.
Substitute:
A poached or very soft-boiled egg comes closest, though the white is then firm and the yolk runny rather than creamy, so it blends less evenly into the dish.